lost 450 gig hard driver

I thought I was successful in returning my Inspiron 530 desktop running Vista then but since upgraded to Windows 7 but now find that I have lost 450 gigs of C drive. I am sure I have seen the answer here before but please help! When I restored the PC from the disc the computer did not find the drivers but they were saved to the external drive so I installed them from there. I had no option to change the size of disc or partition.